Real World Example
Due to popular demand, InetDaemon has written an operational example of how the OSI model can be used to describe what happens when you browse a web page on a web site. This page you are reading is stored on our website and you are most probably using a web browser to read this. We will use the OSI model to describe what happens when you click a link in a web page in your web browser and what happens as the web server receives, processes and responds to your request. This example assumes that you are on a local area network and that you are using an Ethernet card to communicate with the network. If you're on a dial-up modem, it will work a little differently from the data link layer down. Modem communication is a bit more complicated. We won't cover that here.
| LAYER | EXAMPLE | FUNCTION/ACTIVITY | |
| APPLICATION | WebBrowser | A web browser such as Internet Explorer or Netscape provides the means for your computer to contact a web server and download several files that go together to produce a single web page. You can request a web page by typing in a web address (a URL) or by clicking a link in an open web page. The web browser is an APPLICATION. The web browser application gives you the means to select a web server, contact the server and request a web page. The web browser handles the process of finding the web server (the remote computer that has the web page you want stored on it) , requesting the desired web page and displaying all the files contained within the web page. |
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| PRESENTATION | HTTP | The web browser handles PRESENTATION of the web page to the user by converting the files stored at the web server into formats used to display them on your computer. Your web browser supports varous text formats (UNICODE and ASCII), image file formats (JPG, GIF, PNG, BMP), audio file formats (WAV, MP3, AIFF) as well as Hypertext Markup Language (HTML) and one or more scripting languages (JavaScript, JScript, VBScript). Conversion of data from one format to another is the job of the PRESENTATION layer. A web browser can convert these file formats into the local formats used on the local computer for displaying images, playing sounds and displaying text; if it cannot, it often can launch an application which does understand the format. Much of the PRESENTATION layer conversions are handled in the program you're running. |
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| SESSION | When you request a web page, a the web browser opens a TCP connection to the web server. The web server sends back the web page and closes the connection. Your web browser then parses the HTML of the web page. Within the web page are instructions written in HTML tags which tell the browser where to find additional files to be displayed within the web page such as style sheets, sound files, images, movies, Flash files and applets. Your web browser automatically opens additional TCP connections to the web server. Each TCP connection is a SESSION. |
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| TRANSPORT | TCP | To communicate with a web server your computer must open a TCP connection to the web server and request a web page. The TCP connection breaks up theweb page into managable chunks, lables them with numbers so they can be reassembled in the correct order and TRANSPORTS the pieces across the correct SESSION. |
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| NETWORK | IP ARP | Internet Protocol (IP) is a NETWORK layer protocol that uses unique addresses for the web server and for your computer. IP provides the means for your computer to determine whether the web server is a local computer or a computer located somewhere on the Internet. To reach a web server on the Internet, IP protocol also allows your computer to figure out how to reach the Internet web server via your default gateway. Your computer creates a message addressed to the web server with your computer's return IP address. Your computer uses ARP to figure out the physical MAC address of the default gateway and then passes the data to the NETWORK layer. |
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| DATA LINK | E T H E R N E T |
LLC | Once the request from your web browser has been created it is sent to the network card. Once it reaches your network card it must be converted into a message that is sent from your computer to the default gateway which will forward the message to the Internet. At the DATA LINK layer, the web request is inserted inside a network request to the default gateway. |
| MAC | |||
| PHYSICAL | CSMA/CD | The physical layer provides the means to transmit the web page request to the default gateway. |
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Keep in mind that this entire system didn't spring into existence in a day, it took decades for dozens of developments to come together to make the process of requesting, retreiving, delivering and displaying web pages possible.
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